The instant invention relates to the shielding of electrical conductors against the ingress and egress of electromagnetic radiation and more particularly to a novel electrical shielding tape and a novel shielded electrical cable constructed therewith.
The use of metallic shielding tapes in shielded cable constructions is generally known in the electrical industry, and in this regard, reference is made to the applicant's copending U.S. patent application No. 541,361 filed on Oct. 11, 1983 and entitled "Electrical Shielding Tape and Shielded Electrical Cable Constructed Therewith". Further in this regard, a variety of other different electrical shielding tapes have been used in shielded cable constructions for various applications. Generally, the known shielding tapes have been applied in various wrapped dispositions around conductor wires in cables for providing the desired shielding effects against electromagnetic radiation, and external casings or the like have been applied in the cables over the tapes for providing protection against physical damage. In most applications, shielding tapes have been applied to cables in either longitudinally extending "cigarette" wrapped dispositions or in spirally wrapped dispositions, the important point in either case being to provide complete continuous metallic shields around the conductor wires in the respective cables. In most cases, metallic shielding tape constructions have comprised laminations of relatively thin metallic foil layers for shielding, such as aluminum foil layers, and insulative polyester film backing layers which have provided strength and durability in the tapes. Tapes of this type have generally been applied around insulated conductor wires with the foil layers of the tapes facing inwardly, and with the insulative backing layers thereof facing outwardly so that the outer surfaces of the tapes are insulated by the polyester layers. Generally, shielding tapes of this type have been adhesively bonded around inner insulated conductors in shielded cable constructions to firmly secure the tapes in the respective cables, as well as to seal the conductors against moisture penetration which might otherwise take place as a result of capillary action or "wicking" effects. However, one problem which has been experienced in shielded cable constructions of the type comprising foil shielding tapes is the fact that it is difficult to electrically connect foil tapes to ground terminals at the ends of cables, which is necessary in order to achieve the desired shielding effects. While various types of terminal connectors have been available for connection to the shielding layers of shielded cables, they have generally not been effective for making reliable electrical contacts to the thin foil layers of shielding tapes. In order to overcome this problem, some tape-shielded cables have included longitudinally extending uninsulated drain wires which are positioned in physical contact with the inner surfaces of the respective tapes substantially along the entire extents thereof. These drain wires have been provided principally for simplifying electrical interconnections between foil shielding tapes in cables and ground terminals, since drain wires are easily connectable to terminals by conventional means. Unfortunately, however, it has been found that virtually all of the adhesives used for securing shielding tapes in shielded cables have characteristically functioned as electrical insulators, and therefore in cable constructions wherein shielding tapes have been adhesively bonded around conductors, electrical contact between the tapes and the respective drain wires has been hampered by the insulative effects of these adhesives. Accordingly, reliable grounding connections have not always been possible, even in foil shielded cable constructions which have included drain wires.
The instant invention provides a novel solution to the problem of achieving a reliable ground connection in a foil shielded cable by providing a foil shielding tape which is constructed so that it can be adhesively secured in a cable in a manner which provides effective electrical contact with a drain wire in the cable. The electrical shielding tape of the instant invention comprises a metallic shielding layer having an adhesive layer on the inwardly disposed surface thereof for securing the tape in a cable. The adhesive is, however, disposed on the surface in an interrupted layer which defines both exposed contact areas or pads and coated bonding areas thereon; and accordingly, when the tape is used in a cable construction, the bonding areas effectively secure the tape in the cable, but electrical contact between the tape and a drain wire in the cable is provided in the exposed contact pads on the metallic surface of the tape. Preferably the exposed contact pads are disposed in a substantially uniform pattern of contact pads or localized open patches which are on the metallic surfaces discrete areas which are surrounded by adhesive, such as a pattern defined by a plurality of adjacent rows of spaced contact pads wherein the contact pads in adjacent rows are in staggered spaced relation, or a pattern defined by a plurality of spaced rows of spaced contact pads. As a result, the longitudinal penetration of moisture along the cable through wicking effects or capillary action is avoided, because the adhesive effectively seals the cable against such longitudinal penetration. However, effective electrical contact can be reliably achieved in the contact pads between the tape and the drain wire. Accordingly, it is seen that the shielding tape of the instant invention and the shielded cable of the instant invention constructed therewith effectively solve the problem of grounding a foil shielding tape in a reliable manner.
Accordingly, it is a primary object of the instant invention to provide a metallic foil shielding tape which can be adhesively secured in a shielded cable construction so that it is in effective electrical contact with a drain wire in the cable.
Another object of the instant invention is to provide a shielded cable construction of the type having a metallic foil shielding tape which is adhesively secured therein, wherein the shielding tape can be reliably connected to a ground terminal.
Another object of the instant invention is to provide a shielded cable construction, of the type having a foil shielding tape, which can be effectively grounded and which is not susceptible to moisture penetration.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.